1. Field of the Invention
The present invention relates to fault current limiters, and more particularly to a fault current limiter using a superconducting coil.
2. Description of the Background Art
A fault current limiter is used in an electric power circuit for instantly restricting excessive current flow caused by a fault or accident such as a short-circuit. Recently, a fault current limiter using a superconductor has been developed. The fault current limiter using the superconductor utilizes a mechanism for bringing the superconductor from a superconducting state to a normal conducting state in case of an accident such as a short-circuit.
A structure of a superconducting fault current limiter of an induction type is disclosed for example in T. Okazaki, P. D. Evans, xe2x80x9cA Fault Current Limiter in Toroidal Form to Maximize Effective Jcxe2x80x9d, Proceedings of the 1998 Applied Superconductivity Conference, California, Sep. 13-18, 1998. The superconducting fault current limiter of the induction type includes a primary coil having a normal conductor connected to an electric power system and a secondary coil having a superconductor with both ends short-circuited. The secondary coil is designed to be in the superconducting state during normal operation. In this state, a magnetic flux generated by the primary coil is cancelled by that generated by an induced current flowing through the secondary coil. If excessive current flows through the primary coil due to accidental short circuit or the like, the current flowing through the secondary coil also increases. When the superconductor of the secondary coil quenches due to the excessive current flow, a quenching resistance (a resistance after transition to the normal conducting state) is generated in the secondary coil. Thus, an induced current flowing through the secondary coil decreases. Accordingly, the magnetic flux generated by the primary coil is not sufficiently cancelled, and an impedance of the fault current limiter increases. The increased impedance restricts current flow generated in case of the accident.
The superconductor used for the superconducting fault current limiter operating as described above is required to have a high critical current value during normal operation and a high quenching resistance value in case of accidental short circuit or the like (when excessive current flow is caused). For the superconducting fault current limiter of the induction type which is disclosed in the above mentioned reference, it is proposed that a plain coil is arranged in a toroidal form. A primary coil is formed of a pancake coil including a copper wire of a normal conductor, whereas a secondary coil is formed of a ring including oxide superconductors of thin films provided on either side of a zirconia substrate. The primary and secondary coils are alternately arranged such that a coil axis is circular. Thus, the coils are arranged in the toroidal form. Such structure allows a magnetic field to be parallel with a surface of the superconductor and a critical current value of the superconductor to be maximum during normal operation. When excessive current flow is caused due to an accident or the like, the generated magnetic field is perpendicular to the surface of the superconductor and a resistance value of the superconductor is maximum. Thus, an amount of the superconductor required for a designed current value during normal operation becomes minimum, and the resistance value of the superconductor in the case of the accident increases. As a result, excessive current flow due to the accident in the fault current limiter of the induction type is effectively prevented.
In Japanese Patent Laying-Open No. 2-101926, a structure of a superconducting fault current limiter of a noninduction type is disclosed which has two superconducting coils connected in parallel. In the superconducting fault current limiter, one superconducting coil is brought into a normal conducting state, so that the other superconducting coil generates an inductance for restricting excessive current flow in the case of the accident.
In the conventional superconducting fault current limiter of the induction type as described in the above mentioned reference, however, an iron core must be positioned at axes of the primary and secondary coils arranged in the toroidal form. This is because it is difficult to ensure sufficient impedance in the case of the accident if the iron core is not used in the fault current limiter of the induction type. In the conventional superconducting fault current limiter disclosed in the above mentioned reference, after the secondary coil is brought from the superconducting state into the normal conducting state, a resistance value of the secondary coil increases in accordance with a current value of the primary coil. Assuming that the resistance value of the secondary coil is infinite, the fault current limiter disclosed in the reference can be considered to be a simple inductor. Thus, to ensure moderate impedance in the case of the accident in accordance with Faraday""s law, the inductor must suitably be designed only on the side of the primary coil. As a result, to ensure a prescribed impedance required for restricting excessive current flow in the case of accident, the size of the iron core positioned at the axis of the coil must be increased. This disadvantageously increases a weight of the fault current limiter per se. In addition, an iron core in a prescribed shape must be inserted as axes of a plurality of coils arranged in the toroidal form. This results in a complicated structure of the fault current limiter.
Further, in the superconducting fault current limiter of the induction type disclosed in the reference, the primary and secondary coils are inductively coupled. As a result, ampere turns of the primary and secondary coils do not completely match because of an exciting current or leakage flux. Thus, a magnetic field cannot completely be cancelled.
On the other hand, in a superconducting fault current limiter of a noninduction type disclosed in Japanese Patent Laying-Open No. 2-101926, one of two superconducting coils connected in parallel must always be maintained in the superconducting state. Thus, the fault current limiter must securely be designed to allow a sufficient margin to always maintain one superconducting coil in the superconducting state. In addition, to restrict current flow in the case of the accident by the inductance, a suitable approach must be taken such as to increase the number of turns of the coil such that the inductance attains to an optimum value.
An object of the present invention is to provide a fault current limiter capable of ensuring a sufficient impedance for restricting excessive current flow caused by an accident without using an iron core.
Another object of the present invention is to provide a compact fault current limiter capable of satisfying a canceling condition of a magnetic field as well as possible during normal operation and allowing a small residual impedance during normal operation.
The fault current limiter according to one aspect of the present invention includes first and second superconducting coils including windings of superconducting lines. The first superconducting coil is electrically connected in series with the second superconducting coil. The first and second superconducting coils generate magnetic fields in opposite directions by current flow. The fault current limiter of the present invention further includes a component selected from a group consisting of a resistor and an inductor, the component being electrically connected in parallel with the second superconducting coil.
In the fault current limiter of the present invention, as the first and second superconducting coils generate magnetic fields in opposite directions during normal operation, so that the magnetic field in the direction of a coil axis is cancelled. If excessive current flow is caused by an accident, one of the first and second superconducting coils quenches, and a resistance value thereof increases. At the time, current is shunted and flows through the component electrically connected in parallel with one superconducting coil. Thus, values of current flowing through the first and second superconducting coils differ and the canceling condition of the magnetic field is not satisfied. As a result, the magnetic field is generated in the direction of the coil axis, that is, the magnetic field is generated perpendicularly to a surface of a superconductor. Thus, the resistance value of the quenched superconducting coil further increases. By the magnetic field generated perpendicularly to the surface of the superconductor, the other superconducting coil is also brought into a normal conducting state, thereby resulting in a large quenching resistance by the current flowing through the coil and magnetic field thereof. Thus, an overall impedance for restricting excessive current flow caused by the accident can be determined by the quenching resistance value of the superconducting coil without using an iron core. Unlike the conventional superconducting fault current limiter of an induction type, an overall size of the fault current limiter can be designed without any restriction by Faraday""s law. As the iron core is not used, a compact fault current limiter is achieved with reduced weight.
In the fault current limiter of the present invention, the first superconducting coil is electrically connected in series with the second superconducting coil, so that the same amount of magnetic field can accurately be generated in the direction of the coil axis and the canceling condition of the magnetic field can be satisfied completely. It is a well known fact that the superconductor indicates the largest critical current value when a magnetic field is zero.
As the first superconducting coil is electrically connected in series with the second superconducting coil, the canceling condition of the magnetic field can be satisfied even when a direct current flows.
In one embodiment of the fault current limiter according to the above described first aspect of the present invention, the first and second superconducting coils are preferably arranged in a cylindrical shape such that the coil axis is linear, that is, in a solenoid form. The first and second superconducting coils may be arranged such that the coil axis is a curve.
A still more compact fault current limiter is obtained if the first and second superconducting coils are arranged in a cylindrical shape such that the coil axis is linear.
Preferably, the fault current limiter according to the above mentioned first embodiment further includes a third coil connected in series with the first and second superconducting coils and arranged at an end in the direction of the coil axis.
Such structure allows unevenness of the magnetic field at the end of the solenoid form in the direction of the coil axis to be corrected by the third coil to avoid adverse affect on a current-limiting property.
When the first and second superconducting coils are arranged such that the coil axis is a curve, preferably, the fault current limiter according to the above mentioned one aspect of the present invention further includes a third coil connected in series with the first and second superconducting coils and arranged at the end in the direction of the coil axis and a fourth coil connected in series with the first and second superconducting coils and arranged at a portion having a relatively small curvature of the curve. In this case, the fault current limiter may include only the third coil connected in series with the first and second superconducting coils and arranged at the portion having the relatively small curvature of the curve of the curve.
In this case, the uneven magnetic field at the portion with a small curvature, for example, at a bent portion of the axis, is corrected by the coil arranged at the portion having a small curvature to prevent adverse affect on the current-limiting property.
In the fault current limiter having the above described structure, the third or fourth coil may be a normal conducting coil including a winding of a normal conducting line or a superconducting coil including a winding of a superconducting line.
When the third or fourth coil is a superconducting coil including a winding of a superconducting line, a critical current value of the third or fourth coil is preferably set larger than critical current values of the first and second superconducting coils.
Thus, when excessive current flow is caused by the accident at the end in the direction of the coil axis or at the portion having a relatively small curvature, even if the first and second superconducting coils quench and are brought into the normal conducting state, the third or fourth coil does not quench. Accordingly, the coil arranged at the end in the direction of the coil axis or at the portion having a relatively small curvature is prevented from adversary affecting the current-limiting property. Thus, only a central portion of the first and second superconducting coils arranged in the solenoid form generating a uniform magnetic field affects the current-limiting operation. As a result, an equal current load is applied to each of the first and second superconducting coils, and a uniform magnetic field is obtained by each of the first and second superconducting coils.
Preferably, the third or fourth coil has an ampere turn appropriately the same as those of the first and second superconducting coils.
In this case, if the ampere turn of the third or fourth coil is the same as those of the first and second superconducting coils, the number of the turns of the third or fourth coil may differ from those of the first and second conducting coils and a current value of the third or fourth coil may differ from those of the first and second superconducting coils.
When the third or fourth coil includes a winding of a superconducting line, a cross sectional area of the superconducting line forming the third or fourth coil may be larger than that of the superconducting line forming the first and second superconducting coil.
In this case, the performance of the third or fourth coil arranged at the end in the direction of the coil axis or at the portion having a relatively small curvature can be enhanced. By arranging the superconducting coil which corrects unevenness or disorder of the magnetic field at the end or at the portion having a small curvature as the third or fourth coil, the coil may contribute to the current-limiting operation.
In another embodiment of the fault current limiter according to the above mentioned one aspect of the present invention, the first and second superconducting coils are preferably arranged in a ring such that the coil axis is circular, that is, arranged in a toroidal form.
Preferably, in the fault current limiter according to the above mentioned one aspect of the present invention, the resistance value of the resistor or the inductance of the inductor are adjustable.
As shunting of current to the resistor or inductor from the previously quenched superconducting coil can be controlled, the time at which the canceling condition of the magnetic field fails to be satisfied or a degree thereof can be controlled.
A fault current limiter according to another aspect of the present invention includes first and second superconducting coils including windings of superconducting lines. The second superconducting coil is short-circuited. The second superconducting coil is arranged to generate a magnetic field in a direction opposite to that generated by the first superconducting coil.
As the second superconducting coil is short-circuited and arranged in the fault current limiter according to the above mentioned another aspect, when an alternating current is applied to the first superconducting coil, the first and second superconducting coils generate magnetic fields in opposite directions during normal operation. Thus, the magnetic field in the direction of the coil axis is nearly cancelled. When excessive current flow is caused by an accident, the first superconducting coil is quenched. Thus, an impedance of the fault current limiter for restricting excessive current flow caused by the accident is ensured by a quenching resistance value of the first superconducting coil. Unlike the conventional fault current limiter, an iron core is not necessary for ensuring the impedance. As a result, a compact fault current limiter with reduced weight is obtained.
If a critical current value of the second superconducting coil is smaller than that of the first superconducting coil, the second superconducting coil quenches and its resistance value increases faster than the first superconducting coil when excessive current flow is caused by the accident. This differentiates the current values of the first and second superconducting coils, so that the canceling condition of the magnetic field fails to be satisfied. The magnetic field is generated in the direction of the coil axis of the first and second superconducting coils. As the magnetic field is generated perpendicularly to a surface of a superconductor, the resistance value of the quenched second superconducting coil further increases. The first superconducting coil is also brought into a normal conducting state by the magnetic field generated perpendicularly to the surface of the superconductor. Thus, the first superconducting coil comes to have a larger quenching resistance by the current applied to the coil and the magnetic field thereof. As a result, an overall impedance for restricting excessive current flow caused by the accident can be determined by the quenching resistance value of the superconducting coil without using the iron core.
In the fault current limiter according to the above described one aspect of the present invention, the first and second superconducting coils are connected in series, and a component selected from a group consisting of a resistor and an inductor is electrically connected in parallel with the second superconducting coil. Voltages during normal operation and at the time of the accident are applied to the first and second superconducting coils and the component. Particularly, as excessive current flow and voltage are caused by the accident, a high voltage is also generated at each portion between the first and second superconducting coils and the component. Accordingly, sufficient insulation is required at each portion to prevent the dielectric breakdown when the high voltage is generated by the accident. However, in the fault current limiter according to another aspect, basically, only first superconducting coil is connected to a power supply source, and no resistor or inductor is provided. Thus, only a limited portion is subjected to the high voltage generated by the accident. Thus, as the number of portions which must strictly be designed to ensure insulation of the excessive voltage generated by the accident decreases, the fault current limiter can more freely be designed. In addition, a structure of the fault current limiter can be simplified.
In the fault current limiter according to the above described another aspect, the second superconducting coil may include a plurality of superconducting coils each independently short-circuited.
In the fault current limiter according to the above described another aspect, the second superconducting coil may electrically be connected to the first superconducting coil such that a potential of the second superconducting coil is fixed with respect to the first superconducting coil.
When a potential of the second superconducting coil is not fixed, floating electric charges may accumulate in the second superconducting coil. Such accumulation of floating electric charges makes a magnetic field concentrate around the second superconducting coil. As a result, a breakdown voltage may be caused in the second superconducting coil. Thus, in the fault current limiter according to the above described another aspect, the potential of the second superconducting coil is fixed with respect to the first superconducting coil as described above, so that the above mentioned unexpected breakdown voltage is prevented.
In the fault current limiter according to the above described another aspect, the second superconducting coil may be connected to the first superconducting coil through a component selected from a group consisting of a resistor and an inductor.
In this case, the fixed potential of the second superconducting coil can arbitrarily be set by adjusting the component.
Preferably, in the fault current limiter according to the above described another aspect of the present invention, a resistance value of the resistor or an inductance of the inductor is adjustable.
Thus, shunting of the current from the first quenched superconducting coil to the component can be controlled, so that a time at which the canceling condition of the magnetic field fails to be satisfied and a degree thereof can be controlled.
In the fault current limiter according to the above described another aspect, the second superconducting coil may include a plurality of superconducting coils. The plurality of superconducting coils are electrically connected in series and may form a circuit having one and the other ends, which may be short-circuited.
When excessive current is applied to the circuit having the plurality of superconducting coils at the time of the accident, if one of the plurality of superconducting coils is quenched, a current flowing through the overall circuit can rapidly be reduced. Thus, an intensity of the magnetic field generated by all of the plurality of superconducting coils can surely be reduced. As a result, the canceling condition of the magnetic field in the fault current limiter immediately fails to be satisfied, so that the fault current limiter can surely be operated in a short period of time when the accident is caused.
In the fault current limiter according to the above-described another aspect, the above mentioned circuit may electrically be connected to the first superconducting coil such that a potential of the circuit is fixed with respect to the first superconducting coil.
Thus, the floating electric charges are prevented from locally concentrating in the circuit including the plurality of superconducting coils when the high voltage is generated by the accident. As a result, the problem associated with the dielectric breakdown is prevented in the circuit.
In the fault current limiter according to the above described another aspect, the second superconducting coil may include third to sixth superconducting coils. The third and fourth superconducting coils are electrically connected in series to form a first circuit having one end and the other end. One and the other ends of the first circuit may be short-circuited. The fifth and sixth superconducting coils are electrically connected in series to form a second circuit having one end and the other end. One and the other ends of the second circuit may be short-circuited.
With this structure, the plurality of coils as the second superconducting coils can form a plurality of circuits of the first and second circuits, which are short-circuited. When a portion of the plurality of superconducting coils forming the circuit quenches, the use of the plurality of circuits allows the impact by the quench to be rapidly transferred to another superconducting coil forming the circuit.
When the plurality of superconducting coils of the second superconducting coil form a single circuit, a single potential is determined for the single circuit. On the other hand, when the first superconducting coil includes the plurality of superconducting coils, different potentials are determined for the plurality of superconducting coils. Among combinations of oppositely arranged first and second superconducting coils, some combination has a large potential difference of the first and second superconducting coils, thereby making a design for insulation difficult. However, the above described structure allows a potential to be set for each of the first and second circuits, so that the potential difference of the first and second superconducting coils is prevented from being an excessive value.
In addition, the above described structure allows each of the first and second circuits to be designed separately, so that the fault current limiter can more freely be designed.
In the fault current limiter according to the above mentioned another aspect, the first and second circuits may electrically be connected to the first superconducting coil such that potentials of the first and second circuits are fixed with respect to the first superconducting coil.
Thus, floating electric charges are prevented from locally concentrating in the first and second circuits including the plurality of superconducting coils when the high voltage is generated by the accident. Accordingly, the problem associated with the dielectric breakdown is prevented in the first and second circuits.
In the fault current limiter according to the above described another aspect, the first and second superconducting coils may be arranged in a cylindrical shape such that the coil axis is linear.
In the fault current limiter according to the above described aspect, the first and second superconducting coils may be arranged such that the coil axis is a curve.
In the fault current limiter according to the above described aspect, the first and second superconducting coils may be arranged in a ring such that the coil axis is circular.
In the fault current limiter according to the above described first or another aspect of the present invention, the first and second superconducting coils are alternately arranged.
The above described structure allows the magnetic field to be generated in parallel with and perpendicularly to the surface of the superconductor during normal operation and in the case of the accident, respectively, so that an efficient fault current limiter is achieved. Particularly, the magnetic field is efficiently generated perpendicularly to the surface of the superconductor in the case of the accident, so that a larger resistance value is obtained after transition to the normal conducting state. As a result, a more compact fault current limiter with respect to a required resistance value is achieved.
In the fault current limiter according to the above described one or another aspect of the present invention, the superconducting coil is preferably formed of a superconductor spirally arranged on a plane.
In the fault current limiter according to the above described one or another aspect of the present invention, preferably, the first and second superconducting coils include windings of superconducting lines wound in opposite directions such that the first and second superconducting coils generate magnetic fields in opposite directions.
The first and second superconducting coils include windings of superconducting lines wound in the same direction. The first and second superconducting coils may be arranged to generate magnetic fields in opposite directions.
In this case, only a single type of coil may be prepared by using windings of the superconducting lines wound in the same direction, so that the number of components is reduced.
In the fault current limiter according to the above described one or another aspect of the present invention, a critical current value of the first superconducting coil is preferably higher than that of the second superconducting coil.
Thus, when an excessive current begins to flow in the case of the accident, the canceling condition of the magnetic field fails to be satisfied if the second superconducting coil quenches first. In this case, even if the first superconducting coil quenches first, the canceling condition of the magnetic field does not fail to be satisfied. Consequently, it is not expected that a speed of transition to the quenched state by the magnetic field is accelerated.
The transition to the normal conducting state is caused by a slight transition toward the normal conducting state of the second superconducting coil, so that the current is applied to the component for shunting current. The shunting of current makes the complete canceling condition of the magnetic field fail to be satisfied and, the magnetic field is generated in the direction of the coil axis, that is, in the direction perpendicular to the surface of the superconductor. As a result, a critical current value of the entire superconducting coil decreases and a current-limiting operation is rapidly performed. Unlike local heat generation, the magnetic field can be instantly applied to a large area. The application of the magnetic field can rapidly bring the entire superconductor into the normal conducting state, thereby permitting unevenness in a property of the superconductor which is even worse than in the conventional case.
In the fault current limiter according to the above described one or another aspect, the superconducting line preferably includes an oxide superconductor.
One preferred embodiment of the fault current limiter of the present invention includes first and second superconducting coils including windings of superconducting lines. The first superconducting coil is electrically connected in series with the second superconducting coil. The first and second superconducting coils generate magnetic fields in opposite directions when current is applied. The first and second superconducting coils are arranged in a ring such that a coil axis is circular. The first and second superconducting coils are alternately arranged and formed by oxide superconductors spirally arranged on a plane.
Another preferred embodiment of the fault current limiter of the present invention includes first and second superconducting coils having windings of superconducting lines. The first superconducting coil is electrically connected in series with the second superconducting coil. The first and second superconducting coils generate magnetic fields in opposite directions when current is applied. The first and second superconducting coils are arranged in a cylindrical shape such that the coil axis is linear, that is, in a solenoid form. The first and second superconducting coils are alternately arranged and formed of oxide superconductors spirally arranged on a plane. Further, a third coil is provided which is connected in series with the first and second superconducting coils and arranged at an end in the direction of the coil axis.
Another preferred embodiment of the fault current limiter of the present invention includes first and second superconducting coils including windings of superconducting lines. The first and second superconducting coils are arranged in a cylindrical shape such that a coil axis is linear. The second superconducting coil is short-circuited and arranged to generate a magnetic field in a direction opposite to that of the magnetic field generated by the first superconducting coil. The second superconducting coil includes third to sixth superconducting coils. The third and fourth superconducting coils are electrically connected in series to form a first circuit having one end and the other end. One and the other ends of the circuits are short-circuited. The fifth and sixth superconducting coils are electrically connected in series to form a second circuit having one end and the other end. One and the other ends of the second circuit are short-circuited. The superconducting coils are formed of oxide superconductors spirally arranged on a plane.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.